Raster type printers, which have been implemented with various print engines commonly found in the arts, such as electro-photographic print engines and ink jet print engines, employ half-toning to transform continuous tone image data to print data that can be printed as an array of dots that can be of substantially similar size. For example, 24 bit/pixel continuous tone image data can be half-toned to a plurality of single color one-bit per pixel bitmaps.
Half-toning may employ a screen having a matrix of different threshold values. A screen can be a data set with different print density values equally represented (or with a controlled unequal distribution for gamma-compensated screens). For monochrome printing, the image data is then compared with the screen thresholds at each position. If the image data exceeds the threshold, a dot is printed. Otherwise, that particular location remains unprinted.
Improved appearance can be provided using, for example, a pseudo-random stochastic screen having a “blue noise” characteristic. Such screens tend to have threshold values which are distributed so that adjacent values tend to be very different. Thus, any value or limited range of values will tend to be located at positions that are nicely spaced apart on the matrix. In this example, apparently even but random spacing can be emphasized at very low and high density values in a blue noise screen.
For printing with multiple colors, half-toning presents a particular challenge. For dot-on-dot printing, in which printed locations are printed with one or more dots, a single half-toning screen can be used. For instance, a field of 10% blue would have 10% of locations printed with cyan and magenta ink, while 90% of locations remain unprinted. This has the disadvantage of reduced spatial frequency with respect to methods that distribute dots to different locations. This also tends to give the appearance of darker dots more widely spaced apart, producing a grainy image. The same can be said for clustered dot printing techniques in which different color dots may be printed adjacent to each other or otherwise clustered to create a multi-dot cluster that reads as an intermediate color. Accordingly, it is desirable to print the individual dots at closely spaced separate (non-overlapping) locations, relying on the viewer's eye to integrate the different color dots into the intended color.
By using different screens having the threshold values arranged differently, the dots will tend not to align with each other. However, with uncorrelated screens, the printed patterns of different colors will tend to be randomly located with respect to each other. This can generate some graininess of an image as some dots happen to clump near others or overlap. To reduce this with two colors, an inverted screen can be used for one of the colors. An inverted screen often has values equal to the maximum screen value (less the screen value at the corresponding location on the other screen). Thus, 10% blue is printed by printing cyan dots at all locations where the threshold values of the original screen are 25 or less. Magenta dots are printed at locations of values of 230 and above on the original screen (25 or less on an inverted screen). Inverted screens can be limited in usefulness for several reasons.
First, inverted screens may only be used for two colors. This can be inadequate for most multiple color printing systems. Where image quality is not critical in tri-color Cyan, Magenta, Yellow (CMY) systems, the darker C and M dots may be printed in this way while the less visible yellow dots may be distributed otherwise. For four-color systems employing black ink and for multi-level grayscale printing, the inverted screen may not provide desired image quality.
Second, for two color systems where one color is printed at the lowest value range positions, and another is printed at the highest value range positions, those positions are not relatively well dispersed with respect to each other in a blue noise screen. Although it will not generate overlapping droplets at less than full coverage printing, a high frequency blue noise screen may lead to clumps of adjacent dots. Beyond the random effects leading to such clumping, widely different values are more likely to be adjacent to each other.
For three and four color systems, a shifted screen approach has been employed to avoid pure dot-on-dot printing for some colors. This can lead to increased graininess of the image but often generates unwanted low frequency artifacts that are visible in the printed image. Moire patterns may also be generated.
It can be difficult to achieve substantial uniformity or even distribution of the half-toned dots in dot-on-dot printing devices. Substantial uniformity can be computationally expensive.
Another cause of half-tone pattern graininess is spatial luminance variation. A basic property of the color map which is advantageous to control is how the light level, or luminance, changes throughout the color map. Luminance is a photometric quantity which, in essence, is the effect of radiance on our eyes. Radiance is the physical quantity related to light intensity, i.e., the power of the light spreading out in some solid angle over an area. Luminance is a single scalar, (i.e., the integration of radiance weighted with a curve), which describes how efficiently different wavelengths of light trigger cone receptors in our eyes. Because luminance is a weighted integral of radiance, a linear relationship exists between the two. Brightness, on the other hand, is the subjective visual experience of luminance. Roughly, it is the effect of luminance on the brain. Because its a subjective quantity, the relationship of brightness to luminance is non-linear, approximately a cube root.
In many cases, it is desirable that luminance increase monotonically. In other cases, it is desirable that it to remain fixed. One reason why luminance variation is important is because luminance plays a fundamental role in perceptual psychology, i.e., how we perceive details and shapes in images. Perceptual psychology tells us that in most circumstances, luminance is the dominant axis in color perception. But luminance is also how we determine shape from shading. So, if we want to indicate values on a surface of an object by color mapping it, but we also want to show the shape of the object by shading it, then the color map probably shouldn't have any luminance variations in itself, as these could be perceptually misleading. Therefore, it is important to minimize luminance variation for a given C/M/Y/K input.
What is needed in this art is a method which maximizes the dot coverage and therefore leaves the least amount of white void, and which uses as many primary dots as possible before switching to secondary dots and finally the black dots. What is further needed is a method which achieves this while avoiding abrupt color and half-tone texture transitions for close CMYK inputs.